"If you receive subscription perks but later end your D&D Beyond subscription, you'll lose access to the perks you've collected. If you later resubscribe, your subscription perks will be unlocked again."

https://www.dndbeyond.com/posts/979-don-your-best-looks-with-this-months-subscriber

So if you ever stop a subscription, you can't access any of your stuff? What is still available, and what isn't?

You don't need a subscription to view and use your purchased content.  

But perks that are for subscribers you most likely will not have access to anymore.

Subscriber perks are purely cosmetics including dice skins, and special frames and themes for your character sheets. All of your actually important stuff (content) is still available for use.